Particle Measurement Process and Apparatus

ABSTRACT

A process for measuring particle concentrations in a gas using an ejector for producing an essentially constant sample flow and for efficient mixing of the particle-containing sample and and essentially clean, ionized gas. The invention also relates to an apparatus implementing such process. The process and the apparatus can be utilized for example in measuring particle concentrations in an exhaust system of a combustion engine.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a process for measuring particle concentrations in an aerosol. The invention also relates to an apparatus implementing such process. The process and the apparatus can be utilized for example in measuring particle concentrations in an exhaust system of a combustion engine.

2. Description of the State of the Art

For various reasons, it is important to be able to measure fine particles, especially the ones born in combustion engines. The measurement activities are driven by the increasing concern of the potential health effects of fine particles and the health concerns have lead to fine particle emission limits. In the future the particle exhaust limits will continuously become tighter. Another need for fine particle measurements arises from the development of combustion engines and from real-time monitoring of combustion engines, especially diesel engines. Still another need for fine particle measurements arises from the increasing production and use of nanosized particles in industrial processes, the particles typically having a diameter of less than 100 nm. Both the industrial process monitoring as well as occupational health and safety issues require reliable fine particle measurement.

The size distribution of the diesel engine exhaust particles generally shows three different modes: the nuclei mode consists of particles having a diameter of less than approximately 50 nm, the accumulation mode consists of particles having diameters between 50 nm and 1 μm and in the coarse mode the particle diameter is greater than 1 μm. A majority of the diesel engine exhaust particles is born after the exhaust gases escape from the exhaust pipe and these particles typically belong to the nuclei mode.

A particle trap is typically installed to the combustion engine exhaust pipe to keep the fine particle concentrations below the exhaust limits. The trap requires a frequent regeneration and the trapped particles are combusted by increasing the temperature of the trap and simultaneously feeding excess air to the trap, so that the carbon containing particles are burnt.

The patent publication Fl 118278 B, Dekati Oy, 25 Dec. 2004, relates to a method and a sensor device for determining particle emissions from exhaust gases of a combustion engine substantially during the use in an exhaust pipe system or a corresponding exhaust gas duct, in which method emitted particles contained in the exhaust gases are charged and the particle emissions are determined by measuring the electric charge carried by the emitted particles in said exhaust gas duct. According to the invention, the emitted particles are charged by varying the way of charging or the charging power with respect to time in such a manner that as a result of said charging, emitted particles brought into at least two different electrical charge states are present, wherein the charge of the emitted particles is further determined as a difference value/values measured from the emitted particles brought into said at least two different electrical charge states. The problem of the described method is that the particles are charged by a charger placed inside the exhaust gas duct, where the charged easily gets soiled which soiling shortens the lifetime and reliability of the charger. Varying particle concentration and constant ion generation cause problems in e.g. maintaining constant particle charging. The formation of fine particles in the exhaust duct is a complicated process and making measurement in an environment with a greatly varying mass flow is very difficult. E.g. different fuels and different lubricants affect the particle concentrations and particle properties in the exhaust gas duct as described e.g. in Heejung Jung, et al., The Influence of Engine Lubricating Oil on Diesel Nanoparticle Emissions and Kinetics of Oxidation, SAE International 2003-01-3179, 2003.

Francisco J. Romay, et al., A Sonic Jet Corona Ionizer for Electrostatic Discharge and Aerosol Neutralization, Aerosol Science and Technology, Vol. 20 (1994), pp. 31-41, describes the design of a bipolar corona ionizer using a pair of sonic jet ionizers of different polarities. The ionizer was characterized in terms of ion output and particle generation for several, electrodes, orifice plate materials, and ionizer operating conditions. The sonic jet ionizer appeared to generate more particles than typical free corona ionizers. This might be due to a stronger and more reactive corona discharge. The use of silicon electrodes in the sonic jet ionizer resulted in unacceptable levels of particle formation. Lower particle generation could be achieved by using tungsten carbide electrodes and molybdenum orifice plates.

Patent publication U.S. Pat. No. 6,544,484131, TSI Inc., 8 Apr. 2003, describes a system for analyzing aerosols. The system incorporates a corona discharge ion generator with a positively or negatively charged corona discharge needle formed of platinum or a platinum alloy. A high speed (40-210 meter per second) air flow sweeps the ions away from the corona discharge, and propels the ions into a mixing chamber in a turbulent jet that encounters an aerosol, also provided to the mixing chamber. In one version of the ion generator, the ions are carried into the mixing chamber through an orifice formed in a positively or negatively biased plate. In another alternative, the aerosol droplets are electrostatically generated, and propelled into the mixing chamber as an aerosol jet that confronts the ion jet to enhance a mixing of the charged droplets and the ions. In this version the droplets are advantageously neutralized to leave predominantly singly charged positive and negative particles. The problem with system is that the flow ratio of the aerosol and the clean air may change during the measurement e.g. due to the contamination of the filter. The confront mixing of the ionized air and the aerosol by contrary jets is difficult, because the set-up is very sensitive to jet properties, like jet direction and jet velocity and thus the jets tend to be unstable. Ion losses is such set-up are very high, typically more than 99% and even higher than 99.9%

Patent publication U.S. Pat. No. 3,413,545, Regents of the University of Minnesota, 26 Nov. 1968, describes an electric aerosol particle counting and size distribution system for the 0.01 to 2 micron particle range. An aerosol chamber unit having a gas ionizing device and diffusion chamber imparts a unipolar charge on aerosol particles in proportion to the size of the particles. The charged particles are delivered to a mobility analyzer having a housing with an elongated chamber. A particle collecting electrode projects axially into the chamber above a current -collector and sensor filter connected to an electrometer. Collecting voltages up to 30 kV are used in the system. The mixing in the system is inefficient and thus the ion losses are probably very high, typically more than 99.9%.

Patent application US 2006/0144124 A1, Takeshi Kusaka, et al., 6 Jul. 2006, describes An soluble organic fraction, SOF, measuring system that can continuously measure SOF and a soot measuring system that can continuously measure soot are connected with an exhaust gas line. The soot measuring system comprises a ejector diluter that selectively dilutes either one of the exhaust gas and standard gas whose hydrocarbon concentration is known with diluent gas and extrudes it. A dilution ratio adjusting device can adjust a dilution ratio of the diluter. A soot detector continuously detects soot in the exhaust gas or the standard gas diluted by the diluter. The SOF measuring system can be connected with the diluter so that an exhaust gas analyzer can measure the hydrocarbon concentration in the standard gas diluted by the diluter. The publication does not mention charging the diluting air.

The problems with the prior art systems are: charger soiling, poor mixing of ionized air and aerosol, varying sample flow and high ion losses. All these problems make the prior-art systems unstable especially for the on-line measurement of combustion engine exhaust gas particles. The current systems are also large and cannot be used in e.g. monitoring the particle emissions from diesel vehicles. The large mixing chamber in some prior-art systems also slows down the time response of the measurement result.

SUMMARY OF THE INVENTION

The aim of the current invention is to introduce a process and an apparatus which solves the problems of the prior art.

The important components of the process and the apparatus are defined below.

‘Ejector’ is a device using a jet of gas 1, to withdraw gas 2 from a space. The jet gas 1 flow is called ‘main flow’ and gas 2 flow is called ‘side flow’.

‘Swirl’ is a process to cause gas to move with a twisting or whirling motion.

‘Ion trap’ is a device using a combination of electric or magnetic fields or diffusion that captures ions in a region.

It is advantageous to combine the inventive solutions to solve several problems. Thus soiling can be prevented by using gas flows and the same gas can be used, when ionized, as the main flow of an ejector. Efficient mixing can be achieved by an ejector and short mixing time can be achieved by using high velocity ionized gas flow, advantageously in the ejector main flow. The high flow velocity and efficient mixing dramatically reduce ion losses. Rather than measuring the combustion engine exhaust particle emissions inside the exhaust duct, a constant sample flow is used, which makes the measurement process more stable. All solutions described in this invention make the measurement process more stable.

It is known that if swirling jets are used in confront jet mixing; the end result is less stable than with non-swirling jets. Thus the swirling jets are avoided in such systems, e.g. in the one described in U.S. Pat. No. 6,544,484 B1. The inventor has, however, surprisingly found that if swirling gas flow is used as the main flow in the ejector, the combined flow tends to be more stable. Thus swirling main flow is advantageous for the invented process.

The process for monitoring the particle concentration in a gas uses efficient mixing in an ejector for solving the problems of the prior art systems. The main flow of the ejector consists of essentially clean ionized gas flow. The phrase ‘essentially clean’ means that the particle concentration in the ionized gas is so low that it does not adversely affect the monitoring process. The speed of the clean air is preferably sonic or close to sonic, however lower speeds can be utilized as well. The main flow causes suction to the side flow channel and thus a sample flow from the particle-containing gas is sucked to the monitoring apparatus. The ionized clean gas forms the main flow and the sample flow forms the side flow. The main flow should advantageously be as small as possible. The inventor has surprisingly found that a reasonable suction pressure to the side flow channel can preferably be achieved with main flow to side flow ratio 1:1 and even more preferably with main flow to side flow ratio 1:3. The inventor has also surprisingly found that when an ejector is used as described in the invention, two different phenomena, efficient transfer of momentum and effective particle charging happen in a single process step which is advantageous in shortening the process time and thus reducing the ion losses. The efficient mixing makes it possible to design small measurement apparatuses with fast response time, which is a great advantage when measuring vehicle emissions.

When the particle concentration of the gas is monitored, it is advantageous to produce a essentially constant gas flow through the measurement apparatus. Typically the mass flow in e.g. the exhaust duct of a combustion engine is anything but constant, typically depending on the rotation speed of the engine. Using an ejector for sucking the sample flow from the exhaust duct results an essentially constant side flow, the flow being typically pulse-free, i.e. constant. Such a flow can then be e.g. modulated or switched in a controlled way.

One of the problems of the prior art technology is inefficient mixing of clean air and particle-containing gas. The inventor has found that an ejector is efficient in mixing the main flow and the side flow, especially with swirling main flow. In the ejector the main and side flows are advantageously fed close to each other which greatly improves mixing homogeneity and rate when compared to prior art systems. The instability problems existing with the prior art systems are surprisingly avoided with the ejector. Surprisingly it was also found that the high gas velocity of the main flow and the efficient mixing reduce the ion losses and ion losses prior to particle charging are preferably less than 99% and more preferably less than 90%. As the ion losses are considerably lower than in the prior art systems, also the ionizer power supply needs to be smaller providing economical benefit. For efficient measurement it may be advantageous to switch or modulate the mean flow thus carrying out the measurement in AC-mode rather than in DC-mode. Another embodiment for realizing the AC-mode is to connect a chamber parallel to a constant gas flow and to change the volume of the chamber to create a pulsing flow.

The ions that are not attached to the particles have to be removed after the mixing. The ions are removed by an ion trap; the removal mechanism being either electrical or magnetic field or diffusion, these mechanisms can also be combined. The nuclei mode particles can also be trapped to the ion trap, if the trap voltage is increased to a suitable value, the absolute voltage depending e.g. on the geometry of the ion trap, gas flow speed, etc. The size of the trapped particles can be further increased by increasing the trap voltage. Thus it is possible to carry out the trapping with e.g. three different voltages which provides an indication on the size distribution of the accumulation mode particles which can be advantageously used in estimating e.g. the mass or number concentrations. Measuring the electrical current corresponding to different ion trap voltages allows the estimation of various parameters of the nuclei mode and the accumulation mode, like the particle concentration and the mean particle size. The ion trap voltage can be either switched or modulated between at least two different voltages.

Especially when monitoring hot gas flows, like exhaust gases of a combustion engine, the particle concentration may change in the measurement apparatus as well as the cooling gas generates nuclei mode particles. As explained above, the ion trap can be adjusted to a high enough voltage to trap these nuclei mode particles and thus the particles formed in the measurement apparatus itself do not adversely affect the measurement result. In some cases, however, it may be advantageous not to remove the nuclei mode born in the measurement apparatus as these nuclei provide an indication of the particles born after the exhaust gases exit from the exhaust duct. The birth of such nuclei can be intensified by cooling the ejector mean flow. Thus the ion trap voltage switching or modulation can be used for analyzing various parameters from the exhaust gas of a combustion engine.

A preferred embodiment for ionizing the clean gas is to use corona discharger. The clean gas is advantageously fed with high speed from the close vicinity of the corona discharger so that the corona needle or equivalent is protected by the clean gas flow. The high-speed gas flow also increases ion production compared to a free-space corona discharging and decreases ion losses to the apparatus walls etc.

The electrical corona discharge may produce particles which disturb the measurement when the particle concentration in the measured gas is low. This may be the case e.g. when the process is used to monitor particle concentrations before and after a particle trap typically present in the diesel combustion engine exhaust duct. Such a measurement can provide an indication on the filtering capacity or maintenance need of the particle trap. In order to minimize the particle production by the corona discharge, it is advantageous to manufacture the corona needle and the nozzle surrounding the corona needle from a refractory metal or metal alloy, such as tungsten, rhenium, tantalum, molybdenum, iridium, niobium, ruthenium, hafnium, zirconium, vanadium, chrome, alloy C-103, alloy KBI-3, nickel-chrome alloys, tungsten carbide or similar. It is also advantageous to use dry clean gas flow to minimize particle production in the corona discharge.

Clean gas ionization can also be carried out by sparking, thermal emission, surface charging or ionizing radiation.

Because the side flow of the measurement apparatus can be kept essentially constant, the measurement value can be processed with the value of the mass flow of the particle-containing gas and thus e.g. the emissions from a combustion engine can be calculated.

The electrical charge carried by the particles is monitored by measuring the electrical current escaping with the particles. This current measurement method is known from the prior art and with the current invention it provides a benefit by removing the need of capturing the charged particles.

In addition to a combustion engine monitoring, the present invention can be utilized e.g. in monitoring particles in ventilation systems, industrial processes utilizing nanosized particles and in personal particle measurement apparatuses.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail with reference to the appended principle drawing, in which

FIG. 1 shows an embodiment which clarifies the invented process and apparatus for monitoring the particle concentration in gas;

FIG. 2 shows a detail of the ejector;

FIG. 3 shows an embodiment where the main flow and the side flow are directed to flow to the same direction;

FIG. 4 shows an embodiment for modulating the clean air flow; and

FIG. 5 shows the different functional modules of the process.

For the sake of clarity, the figures only show the details necessary for understanding the invention. The structures and details which are not necessary for understanding the invention and which are obvious for a person skilled in the art have been omitted from the figure in order to emphasize the characteristics of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 5 shows the invented process as separated to different functional blocks. The sample flow NV is sucked from the duct or channel where the particle-containing gas flows via an ejector E. Pressurized air flow PI forms the main flow and an ionizer IG is used to ionize the pressurized air flow. The ionizer IG is preferably based on corona discharging, and the necessary high voltage for the corona discharge is generated with the high voltage source HV electrically isolated with the transformer M. The ionized air flow is mixed with the sample flow and the particles of the sample flow are charged in the charging chamber VK. It is to be understood that the charging chamber is really a functional block and the charging preferably takes place in the ejector essentially at the point where the main flow and the side flow meet each other. The ions not attached to the particles are removed by the ion trap IL before the flow PV escapes from the measurement apparatus. The current escaping from the measurement apparatus is measured by the electrometer EM.

FIG. 1 shows an embodiment where the process is used to monitor the particle concentration from the exhaust duct of a combustion engine and the apparatus can be used to monitor the fine particle emissions from a diesel engine. The measurement apparatus 1 comprises the means 3 for directing the sample flow to the measurement channel 4, means 5 for directing the essentially clean, ionized gas to the measurement channel 4, means 6 for mixing the clean gas flow and the sample flow, means 7 for ionizing the clean gas, at least one charging chamber 8 for charging the particles, at least one ion trap 9 and at least one current measurement unit 10 for measuring the current carried by the particles. Additionally the apparatus 1 comprises means 16 for switching or modulating the essentially clean gas flow as well as means 17 for regulating the temperature of the clean gas. FIG. 1 also shows means 20 for connecting the apparatus 1 to the exhaust duct, means 21 for connecting the apparatus to a pressurized air line, pressurized air chamber, or to a similar gas flow production unit. The embodiment also includes means 22 for controlling the clean air flow, means 23 for filtering the pressurized air and means 24 for directing the pressurized air to the measurement apparatus 1.

The pressurized air is fed from the air source P to the temperature regulator 17, which can either heat or cool the air. The magnetic valve 16 is switched to feed the air to the flow controller 22, so that the flow can be set to a desired value. The flow controller 22 can be e.g. adjustable valve, critical aperture, flowmeter, mass flow controller or similar. The flow controller 22 is connected to a filter 23, which essentially removes the particles from the pressurized air, so that the particle concentration in the pressurized air is remarkably lower than the particle concentration in the sample flow. The clean air is then fed to the measurement apparatus 1 through the connector 24. The pressurized air flows to the center channel 18 of the measurement apparatus and the flow may be stabilized with a restrictor plate 26. The center channel 18 may also be designed and constructed so that the flow will be forced to swirl, which is advantageous for various purposes as described above. The flow then escapes through the nozzle 27. The ionization device 7 ionizes the gas escaping from the nozzle 27. The nozzle 27 is advantageously constructed so that the gas flow escapes from the essential vicinity of the corona needle 28. This helps the corona needle 28 to stay clean and improves the ion production. The high flow velocity in the nozzle 27 reduces ion losses. A high voltage is connected to the corona needle 28 via the electrode 29. The high voltage source 34 is electrically isolated from the other system via the transformer 35 and electrical insulator 33. Electrode 29 is in the same electrical potential as the electrode shield 30. Obviously the polarity of the corona needle and the counter electrode may be switched. The ionized gas flow is fed into the measurement channel 4. Also the sample flow from the duct 2 is fed to the measurement channel 4 through the sampling means 3. The sampling means 3 is preferably the side flow 14 of the ejector 11 shown in FIG. 2. The ionized clean gas flow 13 forms the main flow of the ejector 11 and creates a suction to the side flow channel 14, the side flow value depending essentially only from the geometry of ejector 11 and from the value of the mean flow. In a preferred embodiment the ratio of the mean flow to the side flow is small. The geometry and thus the behavior of the main flow channel 13 can be adjusted e.g. by the design of the electrode shield 30. E.g. the gas velocity or the electrical field intensity can be changed either locally or throughout the whole measurement channel. The ionized clean gas flow and the sample flow is mixed in the mixing section 6 of the measurement channel 4. The mixing section 6 should be understood as a functional block which may or may not be physically separated from the measurement channel 4. The mixing section 6 is designed so that the clean gas and particle containing gas are effectively mixed together in a minimum time. Thus e.g. the as flows can be set to swirling motion for effective and stable mixing process. The particles are charged during the mixing and thus also the ionization chamber 8 should be considered as a functional block and the mixing chamber and ionization chamber may be overlapping functions. The ions not attached to the particles are removed by the ion trap 9. Depending on the mechanical construction of the apparatus 1, like the design of the electrode shield 30 or the design of the apparatus' outer wall 31, depending on the mass flow values and the voltage of the ion trap 9, among other parameters, the ion trap 9 can also be adjusted to trap nuclei mode particles and even the smallest particles in the accumulation mode. A preferred embodiment is to adjust the ion trap voltage, the higher voltage trapping larger particles. The actual voltage of the ion trap depends on the other design ang gas flow parameters, but typically the ion trap voltage area may be 1 to 30 kV.

The charge carried by the ionized particles 32 flowing through the measurement channel 4 can be measured by measuring the net current escaping from the ionization device 7. To be able to measure the small currents, typically at pA level, the ionization device 7 is electrically isolated from the other system by the electrical insulator 33 and isolating transformer 35. The electrometer 10 is assembled between the ionization device 7 and to a point having a galvanic contact with the wall 31 of the measurement channel 4. The first contact of the high voltage source 34 is connected to the electrode of the ionization device 7 via means 25 and the second contact is connected to the amplifier of the current measurement device 10. The other input of the electrometer 10 is connected to the wall 31 of the measurement channel 4, which also has a galvanic contact to the ion trap 9. With such setup, the electrometer 10 measures the charge escaping from the measurement channel via the ionized particles, e.g. measures the escaping current.

The net-like electrodes 36 prevent the free ions existing in the measurement channel 4 from escaping to the duct 2. In the preferred embodiment the net-like electrodes 36 are assembled to both flow apertures of the double-skinned measurement apparatus 1, which is beneficial in preventing the ion escape even though the net-like electrodes are designed to have sufficiently large net apertures, which are not easily blocked by the particles flowing through them. The net like electrodes 36 are however, not absolutely necessary in the apparatus 1, because ion escape can be prevented also e.g. with suitable gas flows.

The flow in the measurement channel 4 can be pulsed by switching or modulating the mean flow of the ejector 11 e.g. by pulsing the magnetic valve 16. The mean flow switching or modulation causes a similar switching or modulation of the side flow as well. And thus the total flow is either switched or modulated in a well-controlled way. This allows the electrometer 10 to work in a virtual AC-mode, which provides a more reliable measurement than the DC-mode. The measurement flow may also be switched or modulated. FIG. 2 shows an embodiment where the sample flow may be modulated by a rotating disc 15 assembled to the sample flow channel 14.

FIG. 3 shows a detail of an embodiment describing a way to organize the sample gas flow and the clean ionized gas flow in the invented apparatus. The ionizing device 7 uses a corona needle 28 to ionize the clean gas flow flowing in the center channel 18. Gas escapes from the nozzle 27, which essentially forms the main channel 13 of the ejector 11. The main flow and the sample flow sucked through channel 14 are effectively mixed in the mixing zone 8. The center channel 18 may be designed to force the gas to a swirling motion which further increases the mixing efficiency. Because the gas residence time in the ionizing device 7 is short and an efficient mixing is carried out in ejector 11, the ion losses in the apparatus are preferably less than 99%, more preferably less than 90% and most preferably less than 80%. This means a considerably improvement to the prior art systems, providing great economical benefits, e.g. to the power supply 34.

FIG. 4 shows a principle drawing of an embodiment where the gas flow modulation is carried out by connecting a variable-volume chamber parallel to the constant clean air flow. An essential part of the structure is a piston or a diaphragm 39, whose movement or position can be varied at a certain range. FIG. 4 a shows the construction in principle while FIG. 4 b shows the formation of a cycling flow 37 as the sum function of the constant flow 38 and the moving piston 39. The figure shows the modulation of the piston by a square-wave and the amplitude of the modulation is set in a way that the minimum value of the cycling flow is zero. Obviously the wave form or the flow amplitude may differ in various ways. In another embodiment the moving part may be a diaphragm similar to an audio element. This simple structure has the advantage of being essentially maintenance-free and by almost free selection of wave forms can be generated by the coil 38. Obviously the reciprocating piston 39 can be realized in various ways.

It is obvious for a person skilled in the art that a tight purity requirement is set to the electrical insulators, especially to the ones whose leaking currents would sum to the current signal of the measurement. In hot environment, like in the exhaust duct of a combustion engine, the increase of the insulator temperature decreases their electrical insulation capability. Due to the low current values the measurement electronics needs to be assembled to the essential vicinity of the measurement electrodes and thus the hot temperatures of the system also increase the temperature of the measurement electronics. This increases e.g. the noise of the measurement electronics. In one embodiment of the current invention these problems are solved by using the same gas flow to prevent the insulators from soiling and overheating and to cool the measurement electronics.

It is possible to produce various embodiments of the invention in accordance with the spirit of the invention. Therefore, the above-presented example must not be interpreted as restrictive to the invention, but the embodiments of the invention can be freely varied within the scope of the inventive features presented in the claims hereinbelow. 

1. Process for monitoring a particle concentration in a particle containing gas comprising providing an essentially particle free gas flow, ionizing the essentially particle free gas, sucking a sample flow from the particle containing gas by an ejector where the ionized essentially particle free gas forms the main flow and the sample flow forms the side flow, mixing an essential part of the sample flow with an essential part of the ionized essentially particle free gas flow and thereby charging at least a fraction of the particles and monitoring the electrical charge carried by the particles.
 2. The process as in claim 1 wherein the main flow to side flow ratio is less than 1:1.
 3. The process as in claim 1 wherein the main flow to side flow ratio is less than 1:3.
 4. The process as in claim 1, wherein the ion losses prior to particle charging are less than 99%.
 5. The process as in claim 1, wherein the ion losses prior o particle charging are less than 90%.
 6. The process as in claim 1, wherein the ions not attached to any particle are removed by an ion trap.
 7. The process as in claim 1, wherein the electrical charge carried by the particles is monitored by measuring the electrical current escaping with the particles.
 8. The process as in claim 1, wherein the essentially particle free, ionized gas flow is switched or modulated.
 9. The process as in claim 6, wherein the ion trap also traps the particles belonging to the nuclei mode.
 10. The process of claim 9, wherein the nuclei mode particles are removed by increasing the voltage of the ion trap.
 11. The process as in claim 6 wherein a. the voltage of the ion trap is switched or modulated between at least two different values; and b. the currents corresponding to the said at least two different values are used to calculate the nuclei mode concentration or the accumulation mode concentration or the mass concentration or the number concentration or the size distribution or the concentration of the particles borne after an exhaust duct, or any combination thereof.
 12. The process as in claim 1, comprising using a measurement signal corresponding to a total flow of the particle-containing gas and the particle concentration of the side flow to calculate the particle concentration in the particle-containing gas.
 13. The process as in claim 1, wherein the essentially particle free gas is ionized with corona discharge.
 14. The process as in claim 1, wherein the process is used for particle monitoring before and after a particle trap.
 15. Apparatus for monitoring particles in a duct or channel, comprising: a. means for ionizing essentially particle free gas; b. at least one ejector; c. means for feeding essentially particle free, ionized gas to the main flow channel of the ejector.
 16. The apparatus of claim 15, comprising means for mixing the essentially particle free, ionized gas and a particle-containing gas.
 17. The apparatus of claim 16, wherein the ion losses before the means for mixing are preferably less than 99% and more preferably less than 90%.
 18. The apparatus of claim 15, comprising an ion trap.
 19. The apparatus of claim 18, comprising means for switching or modulating the ion trap voltage at least between two different values.
 20. The apparatus as in claim 15, comprising means for switching or modulating the essentially particle-free ionized gas flow.
 21. The apparatus as in claim 15, comprising means for controlling the temperature of the essentially particle-free ionized gas flow.
 22. The apparatus as in claim 15, comprising a corona discharger for ionizing the essentially particle-free gas.
 23. The apparatus as in claim 22, comprising means for directing the essentially particle-free gas flow to the vicinity of the corona discharger.
 24. The apparatus as in claim 15, wherein the means for ionizing is electrically isolated from the rest of the system and at least one charge measuring means is arranged to determine a net charge obtained by the emitted particles by measuring the discharging current carried along by them from said means for ionizing which discharging current is measured between said means and a point in a galvanic contact with a wall of the apparatus gas flow channel wall.
 25. The apparatus as in claim 15, comprising means for cooling said at least one means for ionizing and components related thereto, by means of a gas flow. 